1. Field of the Invention
The present invention relates to a passive optical network and in particular to the design of an optical receiver for a head-end station used in such a network.
2. Related Art
The present applicants have developed a bit transport system (BTS) for use in a TPON (telephony on a passive optical network) network. In this network downstream data is broadcast in a TDMA multiplexed frame from a head-end station to a number of terminations connected to the head-end station via a passive optical fibre network. In the return or upstream direction, each termination transmits data in a predetermined time slot so that at the head-end station the data from the different terminations are interleaved to form an upstream TDMA frame of predetermined format. The optical receiver in the head-end station therefore has to cope with interleaved pulses arriving from a number of terminations. The precise number of these pulses will be indeterminate at any given instant, varying with the amount of traffic in the upstream direction. It follows that the head-end station in normal operation has to handle a wide range of mean power levels. In addition, certain fault conditions can result in a laser in one of the terminations transmitting continuously, causing an arbitrary shift in the mean levels received at the head-end station. It is important that the receiver should be capable of continuing to function without such faults significantly disrupting traffic.